This invention relates to gas turbine engines and flow compressors utilized thereon, and relates more particularly to an improved diffuser design for use in conjunction with such compressors which exhaust fluid flow at transonic conditions.
Diffusers, such as annularly radial diffusers disposed about the periphery of the radial exit of a centrifugal compressor, function to diffuse the compressed flow by changing the velocity head thereof to an increased pressure. Thus, a diffuser typical to gas turbine engines has an inlet region receiving flow at transonic conditions, and a downstream portion wherein the flow is at subsonic conditions. For a variety of aerodynamic and mechanical efficiency reasons it is conventional practice to utilize vanes extending across the diffuser space. For instance, the vanes act as walls for intercepting boundary layer flows to prevent recirculation thereof back into the compressor. While utilization of vaneless diffusers have been known to the prior art, their applicability and utility is quite limited in practical situations.
It is well known that at transonic flow conditions near Mach 1 in instances, such as diffusers, wherein flow is bounded, the localized Mach number or flow velocity is highly sensitive to changes in flow per unit of cross-sectional area. Accordingly, abrupt changes of a small magnitude, such as about five percent, of the cross-sectional area of the flow passage drastically changes the localized Mach number thereby setting up shock waves and highly varying pressure fields. Such shock waves produce aerodynamic inefficiencies as well as causing certain undesirable mechanical effects such as stress and vibration in the adjacent impeller. Accordingly, it has been conventional practice to avoid emplacing vanes in the region of the diffuser subject to transonic conditions to avoid such shock waves. In the example of a centrifugal impeller, normally there is a vaneless space in the diffuser throughout the region of the diffuser inlet extending at least approximately ten percent of the radius of the radial exit of the impeller.
It has been found that this vaneless space adjacent the exit of the impeller causes a substantial buildup of boundary layer flow at both walls of the diffuser passage. Further, it is believed that the boundary layer flow tends to recirculate back into the compressor impeller rather than being carried radially outwardly along with the remaining flow through the diffuser simply because the boundary layer flow is of such relatively low velocity that it cannot penetrate the higher pressure downstream therefrom in the diffuser.
Another characteristic of rotary compressors is that the flow leaves the impeller and enters the diffuser with a significant nonuniform distribution of flow velocities. Efficient diffusion requires a general matching of the vane direction relative to this flow velocity distribution, and in particular in centrifugal impellers it is many times advantageous that the vanes of a diffuser have a small negative angle of incidence relative to the localized flow direction. (The sign convention normally utilized for the incidence of the vane is that the incidence becomes more positive with decrease in compressor flow.) Many times in the prior art this has resulted in a relatively complicated stator vane shape in the diffuser in order to produce a desired incidence distribution of the vane relative to the localized flow direction.